1. Field of the Invention
The present invention relates to a process for continuously controlling the hydrocarbon heat content of a partial oxidation unit feed-gas stream where the hydrocarbon heat content of the feed-gas stream is subject to variation. More particularly, the present invention relates to a process for continuously controlling the hydrocarbon heat content of such a partial oxidation unit feed-gas stream using recycle synthesis gas as a moderator.
2. Description of the Background Art
Partial oxidation processes are efficient, environmentally sound methods of producing several useful products, including hydrogen, precursors for oxochemical syntheses and electricity. A variety of feed-gases can be used in partial oxidation processes, including natural gas, refinery off-gases, liquified petroleum gas, etc. These feed-gases typically have different hydrocarbon heat contents per unit volume (hereinafter referred to as "hydrocarbon heat content") and because of this a partial oxidation process is typically designed around a specific feed-gas. As used herein, "hydrocarbon heat content" is defined as the heat evolved after complete combustion of paraffinic, olefinic and aromatic components, and after any water has condensed to the liquid state. In a partial oxidation reaction, the primary components of the gaseous feed stream which contribute useful heat are hydrocarbons. Although hydrogen and carbon monoxide generate heat when combusted with oxygen, in a partial oxidation reaction, hydrogen and carbon monoxide (along with nitrogen and carbon dioxide) are effectively inert and contribute no heat to the partial oxidation reaction. For example, natural gas is slightly deficient in hydrocarbon heat content for the partial oxidation reaction. In this case, excess oxygen is needed to burn some of the synthesis gas which is produced to maintain the desired operating temperature in the partial oxidation unit. This method has shortcomings in that more oxygen is required and more product is consumed. Preheating the natural gas can make up for some of this hydrocarbon heat content deficiency, but not all of it.
If the feed-gas has too high a hydrocarbon heat content per unit volume, e.g., above about 1100 Btu/standard cubic foot (Btu/SCF), a moderator is needed for temperature control to protect the partial oxidation unit from overheating. A moderator is a material which is added to the feed-gas to absorb thermal energy without undergoing significant chemical change. In the past, partial oxidation units have been designed to handle specific hydrocarbon feed-stocks, with known, relatively stable hydrocarbon heat contents. When the hydrocarbon heat content of the feedstock is high enough that a moderator is needed, provision is made to introduce steam, CO.sub.2, or water into the partial oxidation unit on a steady state basis.
If a partial oxidation unit is operated under conditions where the hydrocarbon heat content of the feed-gas stream can change over time (e,g., where the partial oxidation unit is used as a means for disposing of one or more refinery waste gases whose combined hydrocarbon heat content is variable, depending on which units are on-line), the conventional steady state addition of moderator will not provide the desired results, i.e., optimum partial oxidation unit temperature. If the hydrocarbon heat content of the feed-gas stream increases, the moderator will be insufficient, and the resulting feed-gas stream will have an excessive hydrocarbon heat content, leading to undesirably higher operating temperatures in a partial oxidation reactor. If the hydrocarbon heat content of the feed-gas stream decreases, the moderator will be too much and the resulting feed-gas stream will have a deficient hydrocarbon heat content, and lower than optimum partial oxidation operating temperatures. This inability to adapt quickly to the changing hydrocarbon heat content of a feed-gas prevents operators from using a different feed-gas in a partial oxidation unit without making significant changes to process design to accommodate the different feed-gas. It is our understanding that prior to the present invention, there has not been a partial oxidation process which operates at a relatively constant, optimum temperature on a relatively constant oxygen and partial oxidation unit feed-gas flows, all while being operated on a feed-gas stream whose hydrocarbon heat content is variable and not known in advance. What is needed is a reliable, fast-acting (on the order of a few seconds) solution for providing more or less moderator to a system as the hydrocarbon heat content of the feed-gas changes. Having standby high pressure steam, carbon dioxide or water ready to inject into the gasifier feed is not an efficient, reliable means of assuring a moderator the instant it is needed.
Accordingly, the present invention provides a cheap, efficient and reliable means of mixing a moderator or a supplemental high hydrocarbon heat content gas into a feedstock as soon as the hydrocarbon heat content of the feedstock varies from a predetermined desirable level. In addition, the process injects the moderator or the supplemental high hydrocarbon heat content gas in an amount appropriate to deliver a feed-gas with an optimal hydrocarbon heat content to the partial oxidation unit, at a constant oxygen flow rate. An advantage of the present invention is that the process can be operated on a variety of feed-stocks, or combinations thereof, which have varying and unpredictable hydrocarbon feed-stocks without having to make changes in process design.